Abradable metal coatings and process therefor



United tates 3,084,064 ABRADABLE METAL COATENGS AND PROtZESS THEREFORLewis M. Cowden, Indianapolis, and Joseph R. Clark,

Brownsburg, Ind, assignors to Union Carbide Corporation, a corporationof New York No Drawing. Filed Aug. 6, 1959, Ser. No. 831,927 13 Claims.(Ci. ill-71) This invention relates to a nongalling coating material andmore specifically concerns an abradable metal coating and a process forproducing same.

Designers of turbines, for example, are confronted by the formidableproblem of reducing the clearance space between the turbine blade andthe turbine housing. This represents a substantial physical barrier toincreasing the turbine speed. If the clearance is reduced, the amount ofgas that escapes, which represents available energy that has not beenfully utilized, becomes less, leading to an increase in efliciency.

In view of the growing number of turbines for jet engines, the emphasison closer tolerances is increasing. The development of new materialsthat can be applied as coatings to housings and that are adapted toclosely conform to the path of the blade tip with a minimum of clearancewill enable engineers to wring greater speeds from smaller turbines.

We have attempted to minimize the clearance between the blade and thehousing of a turbine by coating the inside diameter of the housing withvarious coating materials of sufiicient thickness to establish contactwith the blade, and then allowing the blade to rotate and cut a paththrough the coating. Up to now, this procedure has been unsatisfactorydue to galling and tearing of the coating under the forces generated bythe rotating blades.

It is, therefore, an important object of the present invention toprovide a novel coating material capable of being applied as a coatingto a suitable base material and adapted to suitably abrade whensubjected to sutficiently high forces along its surfaces.

Another object of the invention is to provide a coating that can flakeoff in relatively uniform particles when subjected to sufficiently highforces along its surface without galling or tearing.

Other objects, features and advantages of the present invention will beapparent from the following detailed description of a preferredembodiment thereof.

The present invention is a novel abradable metal coating that flakes offin powder form when subjected to stifliciently high forces along itssurface. It not only imparts the desired abradable coating properties toa suitable base material, but possesses other beneficial properties suchas oxidation resistance and suitable melting point.

The novel coating of the invention comprises a maior componentconstituting the metal phase of the coating and consisting of one ormore metals, and a minor component consisting of a high meltingfinelydivlded powder substantially uniformly distributed in the metalphase of the major component.

The major component may be composed of one or more metals or alloyshaving desirable coating properties with respect to melting point,corrosion resistance, and oxidation resistance. For example, if arelatively low melting point abradable coating is desired, aluminum ispreferred. For higher melting point coatings. metal such as nickel,stainless steel, Nichrome, Monel or other suitable alloys are preferredas the major component.

Among the materials that have been found to be eminently suitable as thehigh melting dispersed powder are boron nitride, carbon, graphite, andmagnesium oxide. Preferably, the powder should be of a material which isnot melted by the process used to apply the coating.

" atent O 3,084,064 Patented Apr. 2, 1963 ice The ratio of metal phasecontent to high melting point powder content of the coating product willdepend upon the desired end use of the abradable coating. A metal phasehaving incorporated therein between approximately 2% and 20% by weightof finely divided high melting point powder is preferred, althoughhigher and lower amounts of powder may be employed, depending upon theparticular powder and base metal. Generally, a low percentage by weightof powder is dispersed in the metal phase of a relatively soft coatingmaterial, and higher weight percent powder contents are employed incoatings of relatively harder material. For example, when a relativelysoft coating material such as aluminum is used, the higher meltingadditive content is about 2 to 5 weight percent. On the other hand, if acoating of Nichrome is desired, the additive content may be as much as15 to 20%.

An important feature of the invention involves the process of applyingthe abradable coating to the surface of a baseplate. In order to obtaingood bond strength between the abradable coating and the baseplate, anundercoat of metal is first applied to the baseplate. This metalundercoat preferably comprises one or more layers, each about 0.003-0005inch thick. it is conveniently of the same metal as the metal phase ofthe final abradable coating, but it could be of any metal which iscompatible with the adjacent metals in the baseplate and abradablecoating. During the entire coating operation of applying both theundercoat and final abradable layer the baseplate is preferably heatedto about 400600 F. in order to obtain the most satisfactory bondstrength between coating and baseplate.

The coating material can be applied by various processes employing ahigh temperature, high velocity means of application. Such methods aredisclosed, for example, in US. Patents Nos. 2,714,563, 2,861,900 and2,847,555 and US. applications, Serial No. 706,099, filed December 30,1957, now abandoned, Serial No. 706,135, filed December 30, 1957, nowabandoned, Serial No. 850,444, filed November 2, 1959, now US. PatentNo. 3,016,447, and Serial No. 521,041, filed July 11, 1955, now Pat. No.2,920,001.

The preferred process of applying the novel coating comprises feedingthe metal phase material in wire or rod form into the efiluent from ahigh temperature, high velocity burner in the manner described incopending application Serial No. 521,041. This eflluent traveling atvelocities greater than 2000 ft./sec. melts the wire to form finelydivided molten metal particles and then rapidly impinges them on thebaseplate, or part being plated, with such force that a firm bondresults. The high melting point dispersed material is simultaneouslyintroduced, preferably in powder form, into the combustion chamber ofthe burner, is discharged into the burner effluent and mixes with themolten metal phase particles prior to impingement on the baseplate. Asan alternative, the high velocity burner efiluent and molten metalparticles could be directed through a gaseous suspension of high meltingpoint powder particles prior to impingement on the baseplate. Stillanother method involves incorporating the powder into the metal wireprior to introduction into the coating apparatus.

While we do not wish to be limited by the following theory, it isbelieved that the abradable characteristics of our novel coating arecaused by the dispersed material preventing the formation of a solid,dense, strongly cohesive metal phase. For this reason, it is desiredthat the dispersed material not be molten during formation of thecoating. Otherwise, a fused or inter-metallic bond between the metalphase and the dispersed material could be strengthened and form acoating which would gall or tear instead of uniformly flaking. Under ourpreferred coating conditions, some of the high melting point materialapplied to the coating area may not be deposited since it may not bemolten.

A spot test used to determine degree of abradability is to take a blunttool and scratch the surface of the coating. Powdery material consistingof small flakes will be removed from an abradable coating while thesurface of a non-abradable coating will simply be scratched with nometal removal or will gall and tear.

In order to indicate still more fully the nature of the presentinvention, the following examples of typical procedures are set forth,it being understood that these examples are presented as illustrativeonly and that they are not intended to limit the scope of the invention.

EXAMPLE I Aluminum-Boron Nitride Abradabie Coating The plating apparatuscomprised a throat combustion burner having an unconstricted combustionchamber /4- in. ID. and 4-in. long. Acetylene at 160 c.f.h. and oxygenat 230 c.f.h. were fed to the burner, combusted and discharged as a hightemperature effluent having velocity greater than 2000 ft./sec. Aluminumwire having -in. dia. was fed at 13.5 grams/min. into the efiluent fromthe burner where it melted and formed finely-divided metal particleswhich were in turn impinged on a baseplate. This baseplate consisted ofa steel ring 4-in. dia. and %-in. wide which was heated to 400-600 F.This coating operation was continued until a layer of aluminum 0003-0005in. thick was deposited on the inner surface of the ring. The aluminumwire feed was then continued and finely-divided boron nitride powder at2 grams/min. was introduced by means of a 40 c.f.h. nitrogen carrier gasstream into the combustion burner throat. The resulting mixture ofaluminum and boron nitride particles were then impinged on the aluminumundercoat to form a coating about 0.080 in. thick. This outer coatingwas analyzed to contain 97 weight percent aluminum. The coated ring wasmounted in a lathe and rotated at 800 rpm. while a blunt-edged tool wasforced onto the coated surface. The coating abraded satisfactorily andformed small particles which readily flaked off. A similar coating wassuccessfully applied to the compressor housing of a jet engine.

If the boron nitride flow is reduced to less than 1.0 gram/ min. whilemaintaining the other coating conditions constant, the resulting coatingloses its abradability. If the boron nitride flow is increased above 4.0grarns/min., the resulting coating is too weak and unreliable.

EXAMPLE II Aluminum-Graphite Abradable Coating The same apparatus andburner operating conditions were used as described in Example I. Anundercoat of aluminum 0003-0005 in. thick was applied to a heatedbaseplate. The abradable coating was applied using feed rates of 13.5grams/ min. for the in. diameter aluminum wire and 17 grams/min. forgraphite powder. The resulting 0.070-in. thick coating was easilyabraded.

The above described aluminum-boron nitride and aluminum-graphitecoatings are satisfactory for use at relatively low temperatures.

The following examples describe formation of abradable coatings whichretain their useful properties at tem peratures as high as 1400 F. Thesecoatings find utility in the turbine sections of commercial jet engineswhich operate at higher temperatures than the compressor section.

EXAMPLE III Nickel-Chromium-Boron Nitride Abradable Coating The sameapparatus and burner operating conditions were used as described inExample I. An undercoat of nickel 0.0030.00-5 in. thick was applied tothe heated baseplate followed by a second coat of Nichrome V Ni20% Cr)of same thickness. The final abradable coating was applied using feedrates of 25 grams/min. for the 0.063-in. dia. Nichrome V wire and 6.0grams/ min. for the boron nitride powder. The resulting 0.060- in. thickcoating had desirable abradable characteristics up to about 1400" F.

EXAMPLE IV Nic'ke[Chromium-Graphite Abradable Coating The same apparatusand burner operating conditions were used as described in Example I. Anundercoat of nickel 0003-0005 in. thick was applied to a heatedbaseplate. The abradable coating was applied using feed rates of 25grams/min. for the 0.063-in. dia. Nichrome V wire and 14.0 grams/min.for the graphite powder. The resulting 0.050-1'n. thick coating waseasily abraded.

EXAMPLE V Nickel-Chromiron-Magnesium Oxide Abradable Coating The sameapparatus and burner operating conditions were used as described inExample I. An undercoat of nickel 0.0030.005-in. thick was applied to aheated baseplate followed by the abradable coating. Feed rates of 25grams/min. for the .063-in. dia. Nichrome V wire and 10.0 grams/min. forthe magnesium oxide powder were employed. The resulting .O30-in. thickcoating had desirable abradable characteristics.

EXAM PLE VI N ickel-C opper-I ron-G rap]: ite A bradable C ating Thesame apparatus and burner operating conditions were used as described inExample I. An undercoat of nickel 0.0030.005 in. thick was first appliedto a heated baseplate. The abradable coating was applied using feedrates of 27 grams/min. for the .063-in. dia. Monel metal (60% nickel-33%copper-6.5% iron) wire and 8.0 grams/min. for the graphite powder. This.030-in. thick final coating was easily abraded.

EXAMPLE VII Nickel-Chromium-Aluminum-Graphite Coating EXAMPLE VIIIIron-Chromium-Nickel-Graphite Abradabie Cvating The same apparatus andburner operating conditions were used as described in Example I. Anundercoat of nickel 0.003-0.005 in. thick was first applied to a heatedbaseplate. The final coating was applied using feed rates of 24grams/min. of .063-in. dia. Type 302 Stainless Steel (iron-carbon alloycontaining 17-19% Cr and 8.10% Ni) wire and 12 gram/min. graphitepowder. The resulting 0.020-in. thick coating was easily abraded.

EXAMPLE IX Nickel-Carbon Abradable Coating The same apparatus and burneroperating conditions were used as described in Example I. An undercoatof nickel 0.003-0.005-in. thick was first applied to a heated baseplate.An abradable coating was then applied to the undercoat by feeding0.063-in. dia. nickel wire at 17 grams/min. into the high velocityburner efiiuent and passing the molten metal particles through a gaseoussuspension of carbon particles prior to impinging on the baseplate.

It will be understood that modifications and variations may be efiectedwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An article comprising a structure having thereon a coating whichproduces a powder-like scraping when scratched with suflicient forcealong its surface consisting essentially of metallically bonded metalparticles defining a fused metal phase having substantially uniformlyincorporated therein in a non-fused relationship, particles of a highmelting powder in minor amounts between 2 and by weight, there beinglesser amounts of high melting powder in said metal phase when saidmetal phase consists of relatively low melting metal than when saidmetal phase consists of relatively high melting metal.

2. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of aluminum and said highmelting point powder consisting of boron nitride.

3. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of aluminum and said highmelting point powder consisting of graphite.

4. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of an alloy of nickel andchromium and said high melting point powder consisting of boron nitride.

5. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of an alloy of nickel andchromium and said high melting point powder consisting of graphite.

6. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of an alloy of iron, chromiumand nickel and said high melting point powder consisting of graphite.

7. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of an alloy of nickel, copperand iron and said high meling point powder consisting of graphite.

8. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of an alloy of nickel, chromiumand aluminum and said high melting point powder consisting of graphite.

9. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of nickel and said high meltingpoint powder consisting of graphite.

10. An article comprising a structure having thereon a coating accordingto claim 1, said metal phase consisting of an alloy of nickel andchromium and said high melting point powder consisting of magnesiumoxide.

11. A coated body comprising a base member and a metallic coating havinggood antigalling properties, bonded thereto, said coating consisting ofa fused metal phase consisting of interbonded particles of a fused metalhaving incorporated therein dispersed non-fused refractory particles inquantities of from 2 to 20% by weight, said coating being abradable uponthe application of a firm force across the surface thereof.

12. A method of producing an abradable metal coating coating on a basematerial comprising applying to said base material an undercoating of ametal bondably compatible with the coating desired, impinging a highvelocity high temperature stream of molten metal phase coating materialagainst said undercoating to form a firm bond therewith, andsimultaneously introducing about 2 to 20% by weight of a high meltingpoint powder into said stream of metal phase coating material prior toimpingement on the undercoat.

13. A method of producing an abradable metal coating on a base materialcomprising: forming a high velocity stream consisting of moltenparticles of a metal to be deposited, impinging said stream against thesurface of the base material to be coated, forming a second highvelocity stream consisting of molten particles of said metal andunmelted particles of a high melting point powder, impinging said streamat high velocity against the base material to fuse the molten metalparticles into a metallically bonded mass having interspersedtherethrough in an amount between 2 and 20 percent by weight and innon-fused relationship, said particles of high melting point powder, andmaintaining said base material and coating at a temperature between 400F. and 600 F. during said coating method.

References Cited in the file of this patent UNITED STATES PATENTS2,178,529 Calkins et al. Oct. 31, 1939 2,261,228 Cockrum Nov. 4, 19412,694,647 Cole Nov. 16, 1954 2,714,563 Poorman et al Aug. 2, 19552,775,531 Montgomery et al. Dec. 25, 1956 2,861,900 Smith et al Nov. 25,1958 2,903,375 Peras Sept. 8, 1959 2,964,420 Poor-man et a1. Dec. 13,1960 3,016,311 Stackhouse Jan. 9, 1962

12. A METHOD OF PRODUCING AN ABRADABLE METAL COATING COATING ON A BASEMATERIAL COMPRISING APPLYING TO SAID BASE MATERIAL AN UNDERCOATING OF AMETAL BONDABLY COMPATIBLE WITH THE COATING DESIRED, IMPINGING A HIGHVELOCITY HIGH TEMPERATURE STREAM OF MOLTEN METAL PHASE COATING MATERIALAGAINST SAID UNDERCOATING TO FORM A FIRM BOND THEREWITH, ANDSIMULTANEOUSLY INTRODUCING ABOUT 2 TO 20% BY WEIGHT OF A HIGH MELTINGPOINT POWDER INTO SAID STREAM OF METAL PHASE COATING MATERIAL PRIOR TOIMPINGEMENT ON THE UNDERCOAT.